Spindle unit for a machine tool

ABSTRACT

The present relates to a spindle unit for a machine tool having a spindle rotatably held in a housing and a hydraulic tool holding system at a forward end of the spindle and capable of holding a tool on the spindle without a tool holder. The spindle of the spindle unit is provided with a tool receiving bore formed in the forward end portion of the spindle along the rotational axis thereof, one or more pressure chambers disposed around the tool receiving bore in spaced relationship from the tool receiving bore by a thin wall elastically deformable in a radial direction of the spindle, and a working fluid passage formed in the spindle and in fluid communication with the one or more pressure chambers. The spindle unit further includes a pressure adjusting device arranged in the rear portion of the spindle unit and connected to the pressure chamber through the working fluid passage. The pressure adjusting device adjustably changes the pressure of the working fluid within the pressure chamber so that it can elastically deform the thin wall to enlarge or reduce the tool receiving bore in radius, thereby holding or releasing the tool inserted into the tool receiving bore.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a spindle unit for a machinetool having a spindle rotatably supported in a housing, and moreparticularly relates to a spindle unit for a machine tool which isprovided at a forward end of a spindle rotatably supported in a housingwith a hydraulic tool holding system for allowing a tool to be held inthe spindle without a tool holder.

[0003] 2. Description of the Related Art

[0004] In a tool machine, two types of spindle units, which aredifferent from each other in a way of mounting a tool on a spindlerotatably supported in a housing of the spindle unit, can be used. Inone type, a tool holder with a tool held therein is mounted on thespindle in order to mount the tool therein. In the other type, what iscalled a holderless spindle type, a tool holder with a tool held thereinis not mounted on the spindle but only a tool is directly mountedtherein. The holderless type of spindle unit is often used in a machinetool especially for machining a workpiece of complicated shape, such asa rib groove of a mold, by means of a relatively narrow tool having asmall cross section.

[0005] The holderless type of spindle unit includes one having amechanical tool holding system capable of gripping the shank portion ofthe tool by a collet or other means located at a forward end of a drawbar of the spindle unit and one having a hydraulic tool holding systemcapable of holding the shank portion of the tool by deforming a toolreceiving bore for receiving the tool therein, which is formed in theforward end of the spindle unit, by means of a pressure medium (workingfluid) such as hydraulic oil.

[0006] It should be noted that the term “forward” or “front” hereinrefers to a side holding the tool and machining the workpiece or thelike and that the term “rear” herein refers to a side opposed to theside machining the workpiece or the like.

[0007] A mechanical tool holding system is disclosed, for example, inJapanese Unexamined Patent Publication (Kokai) No. 8-174364, whichincludes a spindle formed with a tool receiving bore at a forward endthereof and a draw bar extending through the spindle along the axisthereof. During attachment or detachment of a tool, the mechanical toolholding system grips and releases a shank portion of a tool insertedinto the tool receiving bore by a collet located at the forward end ofthe draw bar.

[0008] A hydraulic tool holding system is disclosed, for example, inJapanese Unexamined Patent Publication (Kokai) No. 62-15043, whichincludes a spindle formed with a tool receiving bore at a forward endthereof and a pressure chamber surrounding the tool receiving bore at adistance. The hydraulic tool holding system changes the pressure of aworking fluid enclosed in the pressure chamber so that it enlarges orreduces the diameter of the tool receiving bore in cross section inorder to attach or detach a tool inserted into the tool receiving boreto/from the spindle.

[0009] Nowadays, the machining of a workpiece increasingly requires amuch higher machining accuracy. Thus, this requirement arouses concernabout factors which can cause degradation of machining accuracy, such asmisalignment between the rotational axis of the spindle and a centeraxis of the tool mounted in the spindle, a change in actual tool lengthcaused by deviation of an axial tool hold position where the spindleholds the tool along the length thereof.

[0010] In the case of an NC (numerical control) machine tool using anautomatic tool changer, the above conventional tool holding systemssuffer from the following drawbacks resulting from deviation of the toolhold position in the mounting of the tool.

[0011] In the tool holding system provided with the collet arranged atthe forward end of the draw bar, the draw bar draws the collet rearwardto grip the tool and this movement of the collet moves the tool in itsaxial direction, thereby to change the tool hold position. This mayresult in deviation of the actual tool length.

[0012] On the other hand, in the hydraulic tool holding system, ahydraulic chuck is secured on the front end face of the spindle by meansof suitable fasteners such as screws, as described, for example, inJapanese Unexamined Patent Publication (Kokai) No. 62-15043. Such ahydraulic tool holding system may rotate eccentrically the tool withrespect to the center axis thereof due to misalignment between therotational axis of the spindle and the center axis of the chuck. Thus,such a hydraulic tool system can adversely affect machining accuracy asa result of eccentric rotation of the tool.

[0013] Further, in order to adjustably change the fluid pressure in thehydraulic chuck secured to the forward end of the spindle, a fluidpressure adjusting screw provided in the hydraulic chuck must be turnedat the forward end of the spindle by a wrench or the like. However,since a table and a workpiece are disposed in an area surrounding theforward end of the spindle, there may be insufficient space for workingcomfortably in the area. Therefore, it is not easy to change or adjustthe fluid pressure, and this may reduce operating efficiency. Inaddition, when the automatic tool changer is used with the machine tool,the mechanical tool holding system is often used in combinationtherewith, because the operation of changing the fluid pressure must beperformed at the forward end of the spindle in the conventionalhydraulic tool holding system as described above. However, use of thehydraulic tool holding system makes it difficult to fully automate atool change operation.

SUMMARY OF THE INVENTION

[0014] It is therefore an object of the invention to provide a spindleunit for a machine tool having a spindle and a holderless type of toolholding system, which can hold a tool so that the rotational axis of thespindle is in alignment with the center axis of the tool when the toolis mounted on the spindle, with substantially no deviation in the axialposition of a tool held by the tool holding system, and facilitatesautomatization of the operations of holding and releasing tools.

[0015] The above object is accomplished by a spindle unit for a machinetool according to the present invention which is configured to form atool receiving bore either directly in the forward end of the spindle orin an adapter fixedly secured to the forward end of the spindle so thatthe center axis of the tool receiving bore is in alignment with therotational axis of the spindle, and to employ a hydraulic tool holdingsystem which allows the fluid pressure of the hydraulic tool holdingsystem to be adjustably changed by an operation performed on the rearside of the spindle, where it is easier to secure sufficient space forthe operation to be comfortably performed.

[0016] In accordance with a first aspect of the present invention, thereis provided a spindle unit for a machine tool having a spindle housing,which includes a spindle having a rotational axis and rotatablysupported in the spindle housing; a tool receiving bore formed in theforward end portion of the spindle and extending along the rotationalaxis of the spindle; one or more pressure chambers disposed around thetool receiving bore in spaced relationship therefrom by a thin wallelastically deformable in a radial direction of the spindle, thepressure chamber being filled with working fluid; a working fluidpassage formed in the spindle and in fluid communication with the one ormore pressure chambers; and a pressure adjusting device arranged in therear portion of the spindle unit and connected to the pressure chamberthrough the working fluid passage, the pressure adjusting deviceadjustably changing the pressure of the working fluid within thepressure chamber so that the pressure adjusting device can elasticallydeform the thin wall to enlarge or reduce the tool receiving bore inradius, thereby holding or releasing the tool inserted into the toolreceiving bore.

[0017] In the aforementioned spindle unit, the spindle may include anadaptor mounted on the forward end of the spindle and the tool bore isformed in the adaptor.

[0018] In one embodiment of the spindle unit, a pressure element inserthole is formed in the rear end portion of the spindle to be in fluidcommunication with the working fluid passage, and the pressure adjustingdevice includes a pressure element inserted into the pressure elementinsert hole for pressurizing the working fluid in the working fluidpassage and a pressure element driving device for driving the pressureelement.

[0019] In this embodiment, preferably, the pressure element is a piston,and a space defined by the pressure element insert hole and the pistonis filled with the working fluid and in fluid communication with theworking fluid passage.

[0020] The pressure element may further include a pressure screw coupledto the piston, and the pressure element insert hole may be formed with athreaded portion for engaging with the pressure screw of the pressureelement.

[0021] In this case, preferably, the pressure element driving deviceincludes a motor having a rotatable wrench for rotating the pressurescrew, an actuator for moving the wrench to engage the wrench with anddisengage it from the pressure screw, and a controller for controllingthe operations of the actuator and the motor. The motor may be aservomotor. More preferably, during a process for holding the tool inthe spindle, the controller operates the servomotor in a first operatingmode in which rotational speed control and position control areperformed, and subsequently operates the servomotor in a secondoperating mode in which torque control is performed.

[0022] The pressure element driving device may be rotatably coupled tothe piston. In this case, preferably, a compression spring is placedwithin the space so as to urge the piston in a direction to increase thepressure of the working fluid in the space.

[0023] A compression spring may placed within the space so as to urgethe piston in a direction to increase the pressure of the working fluidin the space, and the pressure element driving device may be separatefrom the piston and move the piston against an urging force of thecompression spring to release the tool in the tool receiving bore.

[0024] In another embodiment of the spindle unit, a check valve isdisposed at the rear end of the working fluid passage opening at therear end of the spindle, and the pressure adjusting device includes amovable working fluid feeding device able to be seated on the rear endface of the spindle, the working fluid feeding device opening the checkvalve to provide fluid communication between the working fluid passageand the pressure adjusting device when the working fluid feeding deviceis seated on the rear end face, thereby allowing the working fluidfeeding device to control the pressure of the working fluid within thepressure chamber.

[0025] In accordance with a second aspect of the present invention,there is provided a spindle unit for a machine tool having a spindlehousing, which includes a spindle having a rotational axis and rotatablysupported in the spindle housing; an adaptor having a center axis andattached to the forward end of the spindle in a manner to align thecenter axis of the adaptor with the rotational axis of the spindle; atool receiving bore formed in the adaptor and extending along the centeraxis of the adaptor; one or more pressure chambers disposed around thetool receiving bore in spaced relationship therefrom by a thin wallelastically deformable in a radial direction of the adaptor, thepressure chamber filled with working fluid; a working fluid passageformed in the spindle and the adaptor and in fluid communication withthe one or more pressure chambers; and a pressure adjusting devicearranged in the rear portion of the spindle unit and connected to thepressure chamber through the working fluid passage, the pressureadjusting device adjustably changing pressure of the working fluidwithin the pressure chamber so that the pressure adjusting device canelastically deform the thin wall to enlarge or reduce the tool receivingbore in radius, thereby holding or releasing the tool inserted into thetool receiving bore.

[0026] In accordance with a third aspect of the present invention, thereis provided a spindle unit for a machine tool having a spindle housing,which includes a spindle having a rotational axis and rotatablysupported in the spindle housing; a tool receiving bore formed in theforward end portion of the spindle and extending along the rotationalaxis of the spindle; one or more pressure chambers disposed around thetool receiving bore in spaced relationship therefrom by a thin wallelastically deformable in a radial direction of the spindle, thepressure chamber being filled with working fluid; a working fluidpassage formed in the spindle and in fluid communication with the one ormore pressure chambers; and a pressure adjusting device connected to thepressure chamber through the working fluid passage, the pressureadjusting device adjustably changing the pressure of the working fluidwithin the pressure chamber so that the pressure adjusting device canelastically deform the thin wall to enlarge or reduce the tool receivingbore in radius, thereby holding or releasing the tool inserted into thetool receiving bore.

[0027] In the spindle unit according to the present invention, the toolreceiving bore extending along the rotational axis of the spindle isformed in the forward end portion of the spindle of the machine tool andat the same time the pressure chamber is disposed around the toolreceiving bore. This arrangement allows the tool to be held in orreleased from the forward end of the spindle by adjustably changing thefluid pressure within the pressure chamber. Therefore, the spindle unitcan eliminate or reduce deviation of the axial position of the tool heldby the spindle, deviation being one drawback of the conventional toolholding system using a collet. The same effect can be achieved by thealternative spindle unit in which the adaptor preformed with a toolreceiving bore, one or more pressure chambers and a working fluidpassage is fixedly secured to the spindle so as to form a single unit.

[0028] Further, since changes of pressure of the working fluid withinthe pressure chamber are performed by a pressure adjusting devicedisposed in the rear portion of the spindle unit, the pressure changeoperations for holding the tool in or releasing it from the spindle donot have to be performed at the forward end portion of the spindle, inwhich the spindle, the housing for supporting the spindle and otherparts may interfere with the operation due to lack of sufficient spacefor comfortable operation. This facilitates the operation of holding thetool in or releasing it from the spindle.

[0029] The arrangement of the pressure adjusting device at the rearportion of the spindle unit also facilitates automation of the operationfor holding the tool in or releasing it from the spindle. For example,the tool holding or releasing operation can be automated by the pressureelement being inserted into the pressure element insert hole opening tothe rear end face of the spindle and by the pressure element drivingdevice driving the pressure element to pressurize the working fluidwithin the working fluid passage.

[0030] Further, the tool holding or releasing operation can be automatedby the check valve being arranged at the end of the working fluidpassage opening to the rear end face of the spindle so that the movablepressure adjusting device can open the check valve when it is seated onthe rear end face of the spindle and by the pressure adjusting deviceadjustably changing the pressure of the working fluid within the workingfluid passage to change the pressure of the working fluid within thepressure chamber.

[0031] Thus, automated operation of holding the tool in or releasing itfrom the spindle can be achieved by the spindle unit, despite the use ofthe hydraulic tool hold system. Further, a completely automated toolchange operation can be achieved when the spindle unit is used incombination with the automatic tool changer.

[0032] In addition, the tool receiving bore formed in the forward end ofthe spindle can eliminate the need to provide an adaptor with the toolreceiving bore preformed therein. As a result, the tool receiving boreformed in the spindle prevents misalignment between the rotational axisof the spindle and the center axis of the tool receiving bore duringmounting of the adaptor on the spindle and therefore makes it possibleto hold the tool in the proper position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and other objects, features, and advantages of thepresent invention will be made more apparent from the followingdescription of the preferred embodiments thereof with reference to theaccompanying drawings, wherein:

[0034]FIG. 1 is a longitudinal sectional view of an embodiment of aspindle unit of a machine tool according to the present invention;

[0035]FIG. 2A is an enlarged longitudinal sectional view of a forwardend of the spindle shown in FIG. 1;

[0036]FIG. 2B is an enlarged longitudinal sectional view of anotherembodiment of a forward end of the spindle;

[0037]FIG. 3 is a flow chart showing a tool change operation in themachine tool using the spindle unit according to the present inventionshown in FIG. 1;

[0038]FIG. 4 is a flow chart showing the tool hold operation shown inFIG. 3 in more detail;

[0039]FIG. 5 is a flow chart showing the tool release operation shown inFIG. 3 in more detail;

[0040]FIG. 6 is a longitudinal sectional view of a main part of a secondembodiment of a pressure adjusting device applicable to the spindle unitof the machine tool according to the present invention;

[0041]FIG. 7 is a longitudinal sectional view of a main part of a thirdembodiment of a pressure adjusting device applicable to the spindle unitof the machine tool according to the present invention; and

[0042]FIG. 8 is a longitudinal sectional view of a main part of fourthembodiment of a pressure adjusting device applicable to the spindle unitof the machine tool according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Preferred embodiments of the present invention will be nowdescribed with reference to the drawings.

[0044] In the following description, it should be noted that unlessotherwise specified, the term “forward” or “front” refers to a sideholding the tool and machining a workpiece or the like, and that theterm “rear” refers to a side opposed to the side machining the workpieceor the like.

[0045] Referring to FIG. 1, a spindle unit 11 for a machine toolaccording to the present invention includes a spindle 13, a housing 17for rotatably supporting the spindle 13 through bearings 15 respectivelydisposed at the front and rear ends of the spindle 13. The bearings 15are fixedly secured in position on the housing 17 by a shoulder formedtherein and by a bearing keep 19 and bearing keep nuts 21. The spindleunit 11 includes a rotor M1 fixed on the center portion of the outerperipheral surface of the spindle 13 and a stator M2 arranged on theinner peripheral surface of the housing 17. The rotor M1 and the statorM2 are opposed to each other at a distance, so that an electromagneticforce acting between the rotor M1 and the stator M2 can rotate thespindle 13 relative to the housing 17.

[0046] A tool receiving bore 25 of a substantially circular crosssection is formed in the forward end face of the spindle 13 and extendsrearwards from the forward end face along the rotational axis of thespindle 13 in the forward end portion thereof. More particularly, thetool receiving bore 25 is defined by an inner hole of a cylindricalsleeve 27, which is inserted into a concentric hole of a circular crosssection formed in the forward end face of the spindle 13 and which isfixed in the hole by a suitable joining method such as welding.Preferably, after the sleeve 27 is fixed in the hole of the spindle 13,the inner peripheral surface of the sleeve 27 may be machined bygrinding so as to bring the center axis of the tool receiving bore 25(more exactly, the inner hole of the sleeve 27 fixedly secured in thehole of the spindle 13) into line with the rotational axis 23 of thespindle 13. This arrangement makes possible proper mounting of the tool(not shown) on the spindle 13, thereby preventing the tool from rotatingabout an eccentric axis with respect to its center axis during rotation.

[0047] One or more recesses are formed on the outer peripheral surfaceof the sleeve 27. When the sleeve 27 is inserted into the hole formed inthe forward end face of the spindle 13, the one or more recesses of thesleeve 27, in cooperation with the peripheral surface of the hole formedin the forward end face of the spindle 13, define one or more pressurechambers 31 around the inner hole of the sleeve 27 (i.e. the toolreceiving bore 25) in spaced relationship from the tool receiving bore25 by a thin wall 29 of the sleeve 27 deformable in its radialdirection. The one or more pressure chambers 31 are connected to apressure adjusting device 35 disposed in the rear portion of the spindleunit 11 through a working fluid passage 33 extending rearward throughthe spindle 13. It should be noted that seals 37 are disposed to theforward and rear sides of the pressure chamber 31 on an interfacebetween the inner peripheral surface of the hole in the forward end ofthe spindle 13 and the outer peripheral surface of the sleeve 27, inorder to prevent the working fluid enclosed in the pressure chamber 31from leaking out along the interface. The pressure chamber 31 ispreferably constituted by an annular chamber but may be constituted by aplurality of chambers circumferentially spaced apart from each other atequal intervals. The pressure chambers 31 are also preferably arrangedin two ring-like rows spaced apart from each other in the axialdirection of the tool receiving bore 25 as shown in FIG. 2A, but may bearranged in one to three rows or more.

[0048] As shown in FIGS. 1 and 2A, an air passage 39 extends through thebearing keep 19, the spindle 13 and the sleeve 27 in the radialdirection of the spindle unit 11 and opens to the inside of the toolreceiving bore 25. Compressed air is supplied from the exterior of thespindle unit 11 to the air passage 39 and used to clean the inside ofthe tool receiving bore 25 or to check the holding of the tool in thetool receiving bore 25 by monitoring the back pressure of the compressedair.

[0049] Referring to FIG. 1, the pressure adjusting device 35 disposed inthe rear portion of the spindle unit 11 will be now described in detail.

[0050] The pressure adjusting device 35 includes a pressure element fordirectly or indirectly pressurizing the working fluid in the workingfluid passage 33 and a pressure element driving device for driving thepressure element.

[0051] The pressure element is inserted into a pressure element inserthole 41, which is formed in the rear end face of the spindle 13 and incommunication with the working fluid passage 33. The pressure elementalso includes a piston 43 inserted in the pressure element insert hole41 to extend to the working fluid passage 33 and a pressure screw 45coupled to the piston 43. The piston 43 is provided at the forward endthereof with a seal member 47 for preventing the working fluid fromleaking out of the working fluid passage 33. The pressure element inserthole 41 is formed with a threaded portion on the inner peripheralsurface thereof for engaging with the pressure screw 45.

[0052] The pressure element driving device, for example, as shown inFIG. 1, includes a wrench motor 51 having a rotating shaft with a wrench49 (for example, a hexagonal wrench) attached thereto, an actuator 53for moving the wrench motor 51 toward and away from the rear end face ofthe spindle to engage the wrench 49 attached to the wrench motor 51 withor disengage it from an engaging recess (for example, a hexagonalrecess) formed at the rear end of the pressure screw 45 of the pressureelement and a controller 55 for controlling the operations of the wrenchmotor 51 and the actuator 53. The wrench motor 51 is mounted on amovable plate 59, which is movably supported by the actuators 53 such asrod cylinders fixed on a bracket 57 located in the rear portion of thespindle unit 11. This arrangement allows the wrench 49 to engage with ordisengage from the engaging recess formed at the rear end of thepressure element.

[0053] The pressure element driving device further includes a lockingelement 61 for stopping the rotation of the spindle 13 when the wrench49 engages with the engaging recess of the pressure screw 45 of thepressure element in order to drive the same. In the embodiment shown inFIG. 1, the locking element 61 is supported on the side of the movableplate 59 opposing the rear end face of the spindle 13. As a result, whenthe movable plate 59 is moved toward the spindle 13 by the actuator 53,the locking element 61 engages with notches or grooves 63 formed on therear end face of the spindle 13 to lock it, thereby preventing thespindle 13 from rotating. The locking element 61 is preferably supportedthrough elastic bodies 65 such as compression springs on the movableplate 59.

[0054] Although an embodiment has been described above in which theforward end of the spindle 13 is formed integral with the tool receivingbore 25, an adaptor 67 preformed with a tool receiving bore 25, apressure chamber 31 and a working fluid passage 33 may of course befixedly secured to the forward end of the spindle 13 by suitablefasteners such as bolts 69 so as to form a single unit with the spindle13, as shown in FIG. 2B.

[0055] Further, in the case where the tool receiving bore 25 isintegrated with the forward end of the spindle 13, the pressureadjusting device 35 may be disposed on the forward side of the spindle13. Even with this arrangement, misalignment between the rotational axisof the spindle 13 and the center axis of the tool receiving bore 25 canbe eliminated, and thus improved machining accuracy achieved.

[0056] Referring to FIG. 3, the tool change operation of the spindleunit 11 for the machine tool shown in FIG. 1 will now be described.

[0057] When an NC (numerical control) device of the machine toolprovides a tool change instruction to the spindle unit 11 at step 151,the spindle unit 11 stops the spindle 13 in a predetermined rotationalangle position so as to allow the locking element 61 of the pressureelement driving device to engage with the notches or grooves 63 formedon the rear end face of the spindle 13 at step 153. It should be notedthat in the above rotational angle position, the wrench 49 can engagewith the engaging recess of the pressure element. Such rotationalpositioning of the spindle 13 can be achieved, for example, by awell-known technique such as use of a sensor (not shown). Next, at step155, the spindle 13 with the tool mounted therein is moved in directionsof the X-axis, Y-axis and Z-axis and then positioned in an empty spaceof a tool magazine (not shown).

[0058] Next, at step 157, the wrench motor 51 of the pressure elementdriving device is moved down (toward the rear end face of the spindle13) to engage the locking element 61 of the pressure element drivingdevice with the notches or grooves 63 formed on the rear end face of thespindle 13. This engagement locks the spindle 13 to prevent itsrotation. At the same time, the wrench 49 attached to the wrench motor51 engages with the engaging recess formed on the rear end of thepressure screw 45.

[0059] At step 159, the wrench motor 51 is rotated counterclockwise andmoves the pressure element rearwards toward the pressure element drivingdevice within the pressure element insert hole 41 to move the piston 43of the pressure element rearwards within the working fluid passage 33.This movement of the piston 43 decreases the pressure of the workingfluid enclosed in the working fluid passage 33 and the pressure chamber31 communicating therewith, so that the thin wall 29 between thepressure chamber 31 and the tool receiving bore 25, which expandsinwardly into the tool receiving bore, deforms elastically to moveoutwardly in the radial direction of the tool receiving bore 25. As aresult, the tool receiving bore 25 is enlarged in radius to release thetool held therein, and the released tool is stored in the tool magazine.

[0060] Next, at step 161, the spindle without a tool held therein ismoved in directions along the X-axis, Y-axis and Z-axis and positionedat a desired tool store position for insertion of another tool in thetool receiving bore 25.

[0061] At step 163, the wrench motor 51 is rotated clockwise and movesthe pressure element forwards toward the forward end of the spindle 13within the pressure element insert hole 41 to move the piston 43forwards within the working fluid passage 33. This movement increasesthe pressure of the working fluid enclosed in the working fluid passage33 and the pressure chamber 31 communicating therewith, so that the thinwall 29 between the pressure chamber 31 and the tool receiving bore 25deforms elastically to move inwardly in the radial direction of the toolreceiving bore 25. As a result, the tool receiving bore 25 is reduced inradius to hold the tool inserted therein.

[0062] At step 165, when the tool is held in the tool receiving bore 25,the wrench motor 51 of the pressure element driving device is moved up(away from the rear end face of the spindle 13) to disengage the wrench49 attached to the wrench motor 51 from the engaging recess formed onthe rear end of the pressure screw 45. At the same time, the lockingelement 61 of the pressure element driving device, which is inengagement with the notches or grooves 63 formed on the rear end face ofthe spindle 13, disengages from the notches or grooves 63.

[0063] The tool change operation is performed by the above steps, andafter completion of the tool change operation, machining restarts withthe new tool at step 167.

[0064] The wrench motor 51 shown in FIG. 1 for use in the embodiment ofthe spindle unit 11 for the machine tool according to the presentinvention requires three types of control: (1) rotational speed controlfor controlling the rotational speed of the wrench motor 51 to preventdamage of the pressure element due to excessively high speed rotation ofthe pressure screw 45; (2) position control of the rotational angle forrotationally positioning the pressure screw 45 of the pressure elementin a predetermined range of rotational angle; and (3) torque control formaintaining the working fluid passage 33 and the pressure chamber 31 ata predetermined pressure. However, a servomotor, which is generally usedas the wrench motor 51, cannot perform these three types of control atonce. Consequently, in order to implement the required control, theservomotor in the above embodiment utilizes its built-in function, i.e.a “torque limit function”, which generates a signal when the torque inthe servomotor reaches a preset level, thereby switching to either oftwo operating modes, a first operating mode for performing therotational speed control and the position control at once and a secondoperating mode for performing only the torque control.

[0065] Referring to FIGS. 4 and 5, the steps of the clockwise rotationand the counterclockwise rotation of the wrench motor 51 controlledabove will be now described in more detail.

[0066] Firstly, referring to FIG. 4, the step of clockwise rotation ofthe wrench motor 51, i.e. the tool hold process for holding the tool inthe spindle 13, will be described. The tool hold process is achieved bytwo separate processes, a standard tightening process and a secondarytightening process. Assume that a first set torque and a first setrotational speed at the standard tightening process, as well as a secondset torque and a second set rotational speed at the secondary tighteningprocess, are predetermined.

[0067] Firstly, when the standard tightening process of the tool holdprocess starts, the wrench motor 51 is switched to the first operatingmode by the controller 55 to enable the rotational speed control and theposition control, and a predetermined torque level (the first settorque) is preset as a torque limit for the standard tightening of thetool hold process. At step 169, the wrench motor 51 is then rotatedclockwise (i.e. in a direction to tighten a screw) at the first setrotational speed (for example, at a rotational speed where the pressurescrew 45 may be moved at 1000 mm/min). At step 171, when the torquelevel of the wrench motor 51 reaches the first set torque, the wrenchmotor 51 (servomotor) generates a torque limit signal by its torquelimit function to stop its rotation. At step 173, the controller 55 thenchecks that the rotational angle of the wrench motor 51 or the positionof the pressure screw 45 falls within a predetermined range.

[0068] For example, if the rotational angle of the wrench motor 51 isless than the minimum limit of the predetermined range, galling of thepressure screw 45 may be suspected, or otherwise if the rotation angleof the wrench motor exceeds the maximum limit of the predeterminedrange, this may be due to insufficient engagement causing slippingbetween the wrench 49 and the engaging recess of the pressure screw 45.Therefore, the wrench motor 51 or the controller 55 preferably generatesa warning signal to indicate an abnormal state when the rotational angleof the wrench motor 51 does not fall within the predetermined range.

[0069] When the result of checking indicates that the rotational angleof the wrench motor 51 is within the predetermined range, the wrenchmotor 51 (servomotor) is switched from the first operating mode to thesecond operating mode to enable torque control. Next, at step 175, thewrench motor 51 restarts clockwise rotation, and stops rotation after apredetermined time (for example, two seconds), at which the torque levelof the wrench motor 51 exceeds the first set torque. In other words, thewrench motor 51 rotates clockwise for the predetermined time under thefirst set torque.

[0070] After completion of the standard tightening process, thesecondary tightening process is then performed. The secondary tighteningprocess is generally similar to the standard tightening process.Firstly, when the secondary tightening process starts, the wrench motor51 is switched again from the second operating mode to the firstoperating mode by the controller 55 to enable rotational speed controland the position control, and a predetermined torque level (the secondset torque) is preset as a torque limit for the secondary tightening. Atstep 177, the wrench motor 51 is then rotated clockwise at the secondset rotation speed (for example, at a rotation speed where the pressurescrew 45 may be moved at 100 mm/min). At step 179, when the torque levelof the wrench motor 51 reaches the second set torque, the wrench motor51 (servomotor) generates a torque limit signal by its torque limitfunction to stop its rotation. At step 181, the controller 55 thenchecks that the rotational angle of the wrench motor 51 or the positionof the pressure screw 45 falls within a predetermined range. When theresult of the check indicates that the rotational angle of the wrenchmotor 51 falls within the predetermined range, the wrench motor 51(servomotor) is switched from the first operating mode to the secondoperating mode to enable torque control. Next, at step 183, the wrenchmotor 51 restarts clockwise rotation. The wrench motor 51 stops rotationafter a predetermined time (for example, two seconds) at which thetorque level of the wrench motor 51 exceeds the second set torque. Atthis point, the secondary tightening process is completed.

[0071] After completion of the secondary tightening process, thecontroller 55 at step 185 stores the information on the rotational angleof the wrench motor 51 prior to movement of the motor 51 away from thespindle 13, in order to ensure engagement of the wrench with theengaging recess of the pressure screw 45 of the pressure element duringthe tool release process.

[0072] Next, referring to FIG. 5, the counterclockwise rotational stepof the wrench motor 51, i.e. the tool release process for releasing thetool from the spindle 13, will be described. Assume that in addition tothe first set torque, the first set rotational speed, the second settorque and the second set rotational speed which are predetermined inthe tool hold process, and a third set rotational speed for use in thefollowing steps are predetermined.

[0073] Firstly, when the tool release process starts, the wrench motor51 is switched to the first operating mode by the controller 55 toenable rotational speed control and position control, and a torque level(the third set torque) higher than the second set torque preset duringthe tool hold process is preset as a torque limit for the tool releaseprocess. At step 187, the wrench motor 51 is then rotatedcounterclockwise (i.e. in a direction to loosen a screw) at the secondset rotational speed. At step 189, after the wrench motor 51 is rotatedby a predetermined rotational angle to move the pressure screw 45 of thepressure element by a predetermined distance toward the rear end face ofthe spindle 13, the controller 55 stops rotation of the wrench motor 51.At step 191, the wrench motor 51 is rotated counterclockwise by apredetermined rotational angle at the third set rotational speed (forexample, at a rotational speed where the pressure screw 45 may be movedat 3000 mm/min). At step 193, when the pressure screw 45 of the pressureelement is moved back to its home position, the controller 55 stopsrotation of the wrench motor 51. The wrench 49 usually does notdisengage from the engaging recess of the pressure screw 45 beforecompletion of the subsequent tool hold process, although the wrench 49can be disengaged from the engaging recess of the pressure screw 45 forreasons of maintenance, etc. Consequently, at step 195, the controller55 stores the information on the rotational angle of the stopped wrenchmotor 51. Thus, the tool release process is completed.

[0074] In the embodiment shown in FIGS. 1 and 2A, the tool receivingbore is defined by the cylindrical sleeve 27, with the recess formed onthe outer peripheral surface thereof being inserted into the bore on theforward end face of the spindle 13. However, as shown in FIG. 2B, aseparate adapter 67, having one or more pressure chambers 31 which aredisposed around the tool receiving bore 25 in spaced relationship fromit by a thin wall 29 deformable in the radial direction of the sleeve27, may be prepared and fixedly secured on the forward end of thespindle 13 so as to form one unit therewith. In this case, the innerperipheral surface of the tool receiving bore 25 is also preferablymachined by grinding to bring the center axis of the tool receiving bore25 into line with the rotational axis of the spindle. The spindle 13 maybe also integrally provided with a tool receiving bore 25 and one ormore pressure chambers 31 disposed around the tool receiving bore 25 inspaced relationship therefrom by a thin wall 25 elastically deformablein the radial direction of the spindle 13.

[0075] Other embodiments of the pressure adjusting device, differentfrom that shown in FIG. 1, are shown in FIGS. 6 to 8.

[0076] A pressure adjusting device 71 shown in FIG. 6 includes apressure element which is inserted in the pressure element insert hole41 formed in the rear end face of the spindle 13 and communicating withthe working fluid passage 33, and a pressure element driving device inthe form of an actuator 75 having a rod 73 which can extend and retractin the axial direction of the spindle 13.

[0077] The pressure element includes a piston 77 and an end cap 79inserted into the pressure element insert hole 41 for closing it. A bore81 is formed on the side of the end cap 79 facing the inside of thespindle 13 and a closing plug is screwed in the opening end of the bore81. The piston 77 is arranged within a space defined by the bore 81 ofthe end cap 79 and the closing plug 83 and divides the space into twoportions. The portion of the divided space positioned on the side of theactuator 75 is in communication with the working fluid passage 33through a connecting passage 85 and is filled with a working fluid.

[0078] A compression spring 87 for urging the piston 77 in a directionto increase the pressure of the working fluid is placed within the otherportion of the divided space by the piston 77, so that the piston 77 canalways apply an urging force to the working fluid within the pressurechamber 31 and the working fluid passage 33 through the space and theconnecting passage 85 to hold the tool in the tool receiving bore 25. Onthe other hand, a rod 77 a extends from the piston 77 through the bottomof the bore 81 of the end cap 79 to the outside of the spindle 13. Thisarrangement makes it possible for the actuator 75 to release the toolfrom the tool receiving bore 25 by the rod 73, separate from the rod 77a of the piston 77 pushing the rod 77 a toward the forward end of thespindle 13 against the urging force of the compression spring 87 todecrease the pressure of the working fluid within the pressure chamber31 through the working fluid passage 33.

[0079] A pressure adjusting device 91 shown in FIG. 7, similar to thatshown in FIG. 6, includes a pressure element which is inserted in thepressure element insert hole 41 formed in the rear end face of thespindle 13 and communicating with the working fluid passage 33, and apressure element driving device in the form of an actuator 95 having arod 93 which can extend and retract in the axial direction of thespindle 13. The pressure element includes a piston 97, which has a rod97 a extending out of the spindle 13 toward the actuator 95 through anend cap 99 attached to the rear end face of the spindle 13 for closingthe pressure element insert hole 41.

[0080] In the pressure element insert hole 41 between the piston 97 andthe end cap 99 is defined a space, within which a compression spring 101for urging the piston 97 forwardly is placed so that the piston 97 canalways apply an urging force to the working fluid within the pressurechamber 31 via the working fluid passage 33, thereby increasing thepressure of the working fluid within the pressure chamber 31 to hold thetool in the tool receiving bore 25. On the other hand, an end of a rod97 a extending from the piston 97 is rotatably coupled to an end of therod 93 of the actuator 95. This arrangement makes it possible for theactuator 95 to release the tool from the tool receiving bore 25 by therod 93 rotatably coupled to the rod 97 a of the piston 97 drawing therod 97 a and the piston 97 toward the actuator 95 against the urgingforce of the compression spring 101 to decrease the pressure of theworking fluid within the pressure chamber 31 through the working fluidpassage 33. It should be noted that the compression spring can generatea pressure in the working fluid within the pressure chamber 31sufficient to hold the tool in the tool receiving bore 25, as in thecase of that of the pressure adjusting device 71 shown in FIG. 6.

[0081] Referring to FIG. 8, a check valve 107 including a spring 103 anda steel ball 105 is disposed at the rear end of the working fluidpassage 33 opening to the rear end face of spindle 13. In other words,the working fluid passage 33 opens to the rear end face of the spindlethrough the check valve 107. More specifically, an end cap 109 isattached to the rear end of the spindle 13 to provide a rear end face ofthe spindle 13 and is formed with a port 111 connected to the workingfluid passage 33 of the spindle 13 and usually closed by the steel ball105 urged rearward by the spring 103. A pressure adjusting device 117 isdisposed behind the rear end face of the spindle 13 and includes aworking fluid feeding device 115 which can be driven forward andrearward by the actuator 113 to be seated on the rear end face of thespindle 13. When the working fluid feeding device 115 is seated on therear end face of the spindle 13, it can open the check valve 107 andprovide fluid communication between the working fluid passage 33 and thepressure adjusting device 117, so that the pressure adjusting device 117can control the pressure of the working fluid within the pressurechamber 31 through the working fluid passage 33.

[0082] The working fluid feeding device 115 shown in FIG. 8 includes ahousing 121 with a cavity 119 formed therein and a valve opening member123 accommodated in the housing 121.

[0083] The cavity 119 of the housing 121 is in fluid communication withthe pressure adjusting device 117 and filled with the working fluid. Awall of the housing 121 facing the rear end face of the spindle 13 isalso formed with a through hole 125, through which the working fluidwithin the cavity 119 can flow to the outside of the housing 121.

[0084] The valve opening member 123 is constantly urged by a compressionspring 127 placed in the cavity 119 to close the through hole 125. Thevalve opening member 123 also includes a projecting portion 123 aextending through the through hole 125 of the housing 121 toward therear end face of the spindle 13. When the working fluid feeding device115 is seated on the rear end face of the spindle 13, the projectingportion 123 a of the valve opening member 123 is inserted into the port111 and pushes back the steel ball 105 of the check valve 107 to openthe check valve 107. At the same time, the urging force applied by thespring 103 of the check valve 107 to the valve opening member 123 movesthe valve opening member 123 in a direction to open the through hole125. This provides fluid communication between the working fluid passage33 and the pressure adjusting device 117 through the cavity 119 in thehousing 121 of the working fluid feeding device 115.

[0085] Thus, when the working fluid feeding device 115 is seated on therear end face of the spindle 13, the pressure adjusting device 117 comesinto fluid communication with the working fluid passage 33, so that thepressure adjusting device 117 can control the pressure of the workingfluid within the pressure chamber 31 through the working passage 33 toperform holding or releasing of the tool inserted in the tool receivingbore 25.

[0086] As described above, the spindle unit according to the presentinvention can eliminate or reduce deviation of the axial position atwhich the spindle holds the tool, and prevent eccentric rotation of thetool due to misalignment of the rotational axis of the spindle with thecenter axis of the tool, thereby to enable highly accurate machining bythe machine tool, and thus overcome drawbacks associated with aconventional tool holding system using a collet. Further, the spindleunit does not require a pressure changing operation for holding the toolin or releasing it from the spindle at the forward end portion of thespindle, in which the spindle, the housing for supporting the spindleand other parts tend to interfere with the operation and there is insufficient space for the operation to be performed comfortably. Thisfacilitates the operation of holding the tool in or releasing it fromthe spindle, thereby making it easy to automate the operation.

[0087] While the present invention has been described with reference tospecific embodiments chosen for the purposes of illustration, it shouldbe apparent that numerous modifications and changes can be made theretoby those skilled in the art without departing from the basic concept andscope of the present invention.

What is claimed is:
 1. A spindle unit for a machine tool having aspindle housing comprising: a spindle having a rotational axis androtatably supported in said spindle housing; a tool receiving boreformed in the forward end portion of said spindle and extending alongthe rotational axis of said spindle; one or more pressure chambersdisposed around said tool receiving bore in spaced relationship fromsaid tool receiving bore by a thin wall elastically deformable in aradial direction of said spindle, said pressure chamber being filledwith working fluid; a working fluid passage formed in said spindle andin fluid communication with said one or more pressure chambers; and apressure adjusting device arranged in the rear portion of said spindleunit and connected to said pressure chamber through said working fluidpassage, said pressure adjusting device adjustably changing the pressureof the working fluid within said pressure chamber so that said pressureadjusting device can elastically deform said thin wall to enlarge orreduce said tool receiving bore in radius, thereby holding or releasingthe tool inserted into said tool receiving bore.
 2. The spindle unitaccording to claim 1, wherein said spindle includes an adaptor mountedon the forward end of said spindle and said tool bore is formed in saidadaptor.
 3. The spindle unit according to claim 1 or 2, wherein apressure element insert hole is formed in the rear end portion of saidspindle to be in fluid communication with said working fluid passage,and wherein said pressure adjusting device comprises a pressure elementinserted into said pressure element insert hole for pressurizing theworking fluid in said working fluid passage and a pressure elementdriving device for driving said pressure element.
 4. The spindle unitaccording to claim 3, wherein said pressure element comprises a pistonand wherein a space defined by said pressure element insert hole andsaid piston is filled with the working fluid and in fluid communicationwith said working fluid passage.
 5. The spindle unit according to claim4, wherein said pressure element further comprises a pressure screwcoupled to said piston and wherein said pressure element insert hole isformed with a threaded portion for engaging with said pressure screw ofsaid pressure element.
 6. The spindle unit according to claim 5, whereinsaid pressure element driving device comprises a motor having arotatable wrench for rotating said pressure screw, an actuator formoving said wrench to engage said wrench with and disengage it from saidpressure screw, and a controller for controlling the operations of saidactuator and said motor.
 7. The spindle unit according to claim 6,wherein said motor comprises a servomotor.
 8. The spindle unit accordingto claim 7, wherein during a process for holding the tool in saidspindle, said controller operates said servomotor in a first operatingmode in which rotational speed control and position control areperformed, and subsequently operates said servomotor in a secondoperating mode in which torque control is performed.
 9. The spindle unitaccording to claim 4, wherein said pressure element driving device isrotatably coupled to said piston.
 10. The spindle unit according toclaim 9, wherein a compression spring is placed within said space so asto urge said piston in a direction to increase the pressure of theworking fluid in said space.
 11. The spindle unit according to claim 4,wherein a compression spring is placed within said space so as to urgesaid piston in a direction to increase the pressure of the working fluidin said space and wherein said pressure element driving device isseparate from said piston and moves said piston against an urging forceof said compression spring to release the tool in said tool receivingbore.
 12. The spindle unit according to claim 1 or 2, wherein a checkvalve is disposed at the rear end of said working fluid passage openingat the rear end of said spindle, and said pressure adjusting devicecomprises a movable working fluid feeding device able to be seated onsaid rear end face of said spindle, said working fluid feeding deviceopening said check valve to provide fluid communication between saidworking fluid passage and said pressure adjusting device when saidworking fluid feeding device is seated on said rear end face, therebyallowing the working fluid feeding device to control the pressure ofsaid working fluid within said pressure chamber.
 13. A spindle unit fora machine tool having a spindle housing, said machine tool comprising: aspindle having a rotational axis and rotatably supported in said spindlehousing; an adaptor having a center axis and attached to the forward endof said spindle in a manner to align the center axis of said adaptorwith the rotational axis of said spindle; a tool receiving bore formedin said adaptor and extending along the center axis of said adaptor; oneor more pressure chambers disposed around said tool receiving bore inspaced relationship from said tool receiving bore by a thin wallelastically deformable in a radial direction of said adaptor, saidpressure chamber being filled with working fluid; a working fluidpassage formed in said spindle and said adaptor and in fluidcommunication with said one or more pressure chambers; and a pressureadjusting device arranged in the rear portion of said spindle unit andconnected to said pressure chamber through said working fluid passage,said pressure adjusting device adjustably changing the pressure of theworking fluid within said pressure chamber so that said pressureadjusting device can elastically deform said thin wall to enlarge orreduce said tool receiving bore in radius, thereby holding or releasingthe tool inserted into said tool receiving bore.
 14. A spindle unit fora machine tool having a spindle housing comprising: a spindle having arotational axis and rotatably supported in said spindle housing; a toolreceiving bore formed in the forward end portion of said spindle andextending along the rotational axis of said spindle; one or morepressure chambers disposed around said tool receiving bore in spacedrelationship from said tool receiving bore by a thin wall elasticallydeformable in a radial direction of said spindle, said pressure chamberbeing filled with working fluid; a working fluid passage formed in saidspindle and in fluid communication with said one or more pressurechambers; and a pressure adjusting device connected to said pressurechamber through said working fluid passage, said pressure adjustingdevice adjustably changing the pressure of the working fluid within saidpressure chamber so that it can elastically deform said thin wall toenlarge or reduce the radius of said tool receiving bore, therebyholding or releasing the tool inserted into said tool receiving bore.